Apple users may find that Siri will give them their birthdays wrong if it’s still to come.
The voice assistant may already assume that the person asking has already had their birthdays. iCulture, a Dutch blog explains the reason why- Siri only takes into account the user’s year of birth, but not the month and exact day. Individuals can test this out by activating Siri, but only if they’ve added their birthdays on their iPhone’s contact card.

The same incorrect estimate holds true for people in the iPhone’s contact list. This only works if the user has added the contact’s birthday information; otherwise, Siri will respond with a statement that the user has not added a birthday to the contact’s card.
The Siri birthday question does not affect all iOS users, and it’s unclear on what factors determine why this is so. The issue might be easily fixed in the next iOS update.